1. Field of the Invention
The present invention relates to a liquid crystal display device capable of performing multi-gradation display. Particularly, the present invention relates to a liquid crystal display device which can be driven by a driver of an existing type and yet can perform multi-gradation display with higher performance than expected with the existing type of driver.
2. Description of the Related Art
A liquid crystal display device and a plasma display device are known as image display devices using a flat panel. Usually, a digital signal is used at the input interface of these display devices. In a display device using a digital signal at its input interface, the number of displayable gradations depends on the number of bits contained in a signal to be used. As the number of gradations increases, the number of bits increases too. In case of a liquid crystal display device, a source driver achieving the largest number of gradations among source drivers now in practical use is an 8-bit type (256 gradations). Gradations more than this can not be displayed.
Suppose that a 12-bit type source driver is developed by simply increasing the number of bits. When such a 12-bit type source driver is compared with an 8-bit type source driver, the number of resistors comprised in a digital-analog converter (hereinafter referred to as DCA) for generating each gradation and the number of switch circuits for selecting the resistors required in the 12-bit type source driver are 16 (2122/8=4096/256=16) times larger than those required in the 8-bit type source driver. Consequently, the size of the circuit becomes considerably large, and an increase in costs is inevitable because of expansion of the chip size. Hence, there occurs an idea of enabling display of a larger number of gradations than achieved by an existing circuit system yet with the use of an existing circuit system. As one method therefore a method of using each unit pixel by dividing it into a plurality of pixels has been proposed.
Unexamined Japanese Patent Application KOKAI Publication No. 2001-34232 proposes one such method. FIG. 16 is a block diagram showing an example of the structure of a liquid crystal display device according to a prior art, to which the present invention will be applied. As shown in FIG. 16, the liquid crystal display device 100 comprises a color liquid crystal panel 101, a backlight 102, a cell driver 103, a data processing unit 104, and an input/output (I/F) unit 105.
The color liquid crystal panel 101 displays a color image by liquid crystal cells arranged on a plane. The backlight 102 is a light source which emits white-color light from the back of the liquid crystal panel, so that the liquid crystal panel may perform color image display by transmissive light. The cell driver 103 generates a drive signal for driving each liquid crystal cell of the liquid crystal panel based on input data. The data processing unit 104 performs data processing for supplying input data to the cell driver 103 in response to an input digital signal. The I/F unit 105 constitutes an interface for external inputting and outputting. The cell driver 103 is built up by a source driver (not shown) and a gate driver (not shown). The source driver controls the source of each transistor for driving each liquid crystal cell along the arrangement in the vertical direction (column direction). The gate driver controls the gate of each transistor along the arrangement in the horizontal direction (row direction).
FIG. 17 are diagrams for explaining an example of a display screen of a conventional liquid crystal display device, which is disclosed in the aforementioned Unexamined Japanese Patent Application KOKAI Publication No. 2001-34232. FIG. 17(A) is a view of the color liquid crystal panel in partial enlargement. FIG. 17(B) is a diagram showing an example of how to divide each unit pixel. As shown in FIG. 17(A), in case of using color filters, the display screen of the color liquid crystal panel 101 of the conventional liquid crystal display device is structured in a way that an R (red) pixel, a G (green) pixel, and a B (blue) pixel are sequentially and repeatedly arranged horizontally in each row. With the use of the color filters, color display is performed through these R pixels, G pixels, and B pixels based on red image data, green image data, and blue image data respectively. However, a monochrome image is displayed in each liquid crystal cell constituting each pixel of the color liquid crystal panel 101.
Specifically, in the color liquid crystal panel 101, one set of an R pixel, a G pixel, and a B pixel is used as a unit pixel and monochrome display is performed in each unit pixel. Since a unit pixel of a color image is constituted by an R pixel, a G pixel, and a B pixel in case of using color filters, the number of brightness levels displayable by one unit pixel is three times as large as the number of brightness levels displayable by each of the R pixel, the G pixel, and the B pixel.
Therefore, it is possible to break the gradation levels of a display image into more minutely-stepped levels by dividing the brightness level range into, for example, three and scale-marking each divided range. Let it be assumed that one unit pixel p is divided into three pixels p1, p2, and p3 as shown in FIG. 17(B), and each of the pixels p1, p2, and p3 performs 8-bit display. Since the brightness level range displayable by each pixel is from 0 to 255, the brightness level range displayable by the unit pixel p is from 0 to 765 (255×3). A display image including a high gradation level can be achieved by arranging that the smallest value 0 in the brightness level range correspond to the smallest value in image data and the largest value 765 in the brightness level range correspond to the largest value in the image data.
When the data processing unit 104 supplies a brightness value converted from image data to the unit pixel p, it divides the value almost equally among the three pixels p1, p2, and p3. Specifically, let a case be considered where 8-bit image data is input to a color display for performing 8-bit display. The 8-bit image data is composed of values of 0 to 255. In this case, it is arranged that the smallest value in the image data correspond to the smallest brightness value 0 of the color display and the largest value in the image data correspond to the largest brightness value 765 of the color display.
FIG. 18 shows a relationship between brightness values of a unit pixel and brightness values of each pixel in a conventional liquid crystal display device. The data processing unit 104 divides a brightness value acquired from image data among the pixels p1, p2, and p3 as shown in FIG. 18. For example, in case of a brightness value 0 for the unit pixel p, the pixels p1, p2, and p3 are given shares of 0, 0, and 0 respectively. In case of a brightness value 1 for the unit pixel p, the pixels p1, p2, and p3 are given shares of 0, 0, and 1 respectively. In case of a brightness value 2 for the unit pixel p, the pixels p1, p2, and p3 are given shares of 0, 1, and 1 respectively. Likewise, until the brightness value 765 for the unit pixel p, the brightness value for each pixel is determined in the same way. To sum up, according to the conventional liquid crystal display device 100 shown in FIG. 17, a brightness value is equal to the gradation level input to the liquid crystal display device 100. According to the prior art, in the liquid crystal display device 100, the unit pixel p is divided into three homogeneous pixels p1, p2, and p3, thereby achieving an almost three-times-larger number of gradations by adding the gradations (input data to the driver) of all the three pixels. FIG. 19 shows a relationship between input gradation levels and brightness values in the conventional liquid crystal display device 100. FIG. 19 reveals that the relationship between gradation levels input to the liquid crystal display device 100 (or data for each pixel input to the driver) and brightness values (standardized brightness values in FIG. 18) is linear. Therefore, the sum of brightness values of all the pixels p1, p2, and p3 is equal to the brightness value of the unit pixel p.
Further, the Publication of Japanese Patent No. 2700903 discloses a technique for regarding a plurality of neighboring pixels as one display unit, controlling the gradation level of each display unit by changing combinations of the lighting and non-lighting states of each pixel in the display unit or the gradation level of each pixel in the display unit, and arranging that the center of the display unit correspond to the center of the densities of the middle tones.
The invention disclosed in the Publication of Japanese Patent No. 2700903 is directed to a liquid crystal display device of a so-called simple matrix type, and for performing gradational display by varying the width of a data electrode.